Exhaust gas aftertreatment systems

ABSTRACT

A system for effective NOx control in a diesel or other lean burn internal combustion engine is presented. The system includes a urea-based SCR catalyst having an oxidation catalyst coupled upstream of it and an ALNC coupled upstream of the oxidation catalyst. This system configuration results in improved NOx conversion due to faster SCR catalyst warm-up and higher operating temperatures. Additionally, placing the ALNC upstream of the oxidation catalyst prevents hydrocarbon slip into the SCR catalyst at low exhaust gas temperatures. Also, system reliability is improved by adding an auxiliary NOx aftertreatment device.

FIELD OF INVENTION

The present invention relates to an emission control system for dieseland other lean-burn vehicles and, more specifically, to a new systemconfiguration designed to achieve improved emission control.

BACKGROUND OF THE INVENTION

Current emission control regulations necessitate the use of catalysts inthe exhaust systems of automotive vehicles in order to convert carbonmonoxide (CO), hydrocarbons (HC), and nitrogen oxides (NOx) producedduring engine operation into unregulated exhaust gasses. Vehiclesequipped with diesel or another lean burn engine offer the benefit ofincreased fuel economy, however, control of NOx emissions in suchsystems is complicated due to the high content of oxygen in the exhaustgas. In this regard, Selective Catalytic Reduction (SCR) catalysts, inwhich NOx is continuously removed through active injection of areductant, such as urea, into the exhaust gas mixture entering thecatalyst, are know to achieve high NOx conversion efficiency. A typicallean exhaust gas aftertreatment system may also include an oxidationcatalyst coupled upstream of the SCR catalyst. The oxidation catalystconverts hydrocarbons (HC), carbon monoxide (CO) and nitrous oxide (NO)in the engine exhaust gas. The oxidation catalyst is also used to supplyheat for fast warm up of the SCR catalyst.

The inventors herein have recognized several disadvantages with suchsystem configuration. Namely, because the oxidation catalyst istypically located under-body far downstream of the engine, it takes asignificant time to reach light-off temperatures (e.g. 200 deg. C.).This results in delayed warm up for the SCR catalyst, and thusnegatively affects emission control. Also, since the oxidation catalystdoes not convert the incoming hydrocarbons until it reaches light-off,under some conditions, such as cold start, or extended periods of lightload operation, hydrocarbons may slip from the oxidation catalyst andcause the SCR catalyst poisoning.

SUMMARY OF THE INVENTION

Accordingly, the present invention is directed to a system that achieveseffective control of NOx emissions in a lean burn engine, such as adiesel engine, while overcoming the disadvantages of the prior art. Thesystem includes an Active Lean NOx catalyst (ALNC), an oxidationcatalyst coupled downstream of the ALNC, and a selective catalyticreduction (SCR) catalyst coupled downstream of said Active Lean NOxcatalyst.

Such system configuration results in decreased oxidation catalystlight-off time due to the exotherm generated by the ALNC. Also, unlikethe oxidation catalyst, the ALNC can store hydrocarbons at low exhaustgas temperatures, therefore SCR catalyst poisoning due to hydrocarbonslip is prevented. Additionally, since the ALNC has NOx conversioncapabilities, demands on the SCR catalyst are less severe, and the ALNCcan serve as an auxiliary NOx aftertreatment device if the SCR catalystperformance becomes degraded.

An advantage of the present invention is improved emission control dueto the reduced emission control system warm-up time. Another advantageof the present invention is improved SCR catalyst durability and NOxconversion efficiency are achieved by eliminating the risk ofhydrocarbon poisoning. Yet another advantage of the present invention isimproved emission control system reliability due to the presence of anadditional NOx aftertreatment device.

The above advantages and other advantages, and features of the presentinvention will be readily apparent from the following detaileddescription of the preferred embodiments when taken in connection withthe accompanying drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and advantages described herein will be more fullyunderstood by reading an example of an embodiment in which the inventionis used to advantage, referred to herein as the Description of PreferredEmbodiment, with reference to the drawings, wherein:

FIGS. 1A and 1B are schematic diagrams of an engine wherein theinvention is used to advantage;

FIG. 2 is a schematic diagram of an exemplary embodiments of an emissioncontrol system in accordance with the present invention;

DESCRIPTION OF PREFERRED EMBODIMENT(S)

Internal combustion engine 10, comprising a plurality of cylinders, onecylinder of which is shown in FIG. 1A, is controlled by electronicengine controller 12. Engine 10 includes combustion chamber 30 andcylinder walls 32 with piston 36 positioned therein and connected tocrankshaft 40. Combustion chamber 30 is shown communicating with intakemanifold 44 and exhaust manifold 48 via respective intake valve 52 andexhaust valve 54. Intake manifold 44 is also shown having fuel injector80 coupled thereto for delivering liquid fuel in proportion to the pulsewidth of signal FPW from controller 12. Both fuel quantity, controlledby signal FPW and injection timing are adjustable. Fuel is delivered tofuel injector 80 by a fuel system (not shown), including a fuel tank,fuel pump, and fuel rail (not shown).

Controller 12 is shown in FIG. 1A as a conventional microcomputerincluding: microprocessor unit 102, input/output ports 104, read-onlymemory 106, random access memory 108, and a conventional data bus.Controller 12 is shown receiving various signals from sensors coupled toengine 10, in addition to those signals previously discussed, including:engine coolant temperature (ECT) from temperature sensor 112 coupled tocooling sleeve 114; a measurement of manifold pressure (MAP) frompressure sensor 116 coupled to intake manifold 44; a measurement (AT) ofmanifold temperature from temperature sensor 117; an engine speed signal(RPM) from engine speed sensor 118 coupled to crankshaft 40.

An emission control system 20 is coupled to an exhaust manifold 48 andseveral exemplary embodiments of the system in accordance with thepresent invention are described with particular reference to FIGS.2A-2C.

Referring now to FIG. 1B, an alternative embodiment is shown whereengine 10 is a direct injection engine with injector 80 located toinject fuel directly into cylinder 30.

Referring now to FIG. 2, the emission control system 20 includes anActive Lean NOx catalyst (ALNC) 13, an oxidation catalyst 14, aurea-based Selective Catalytic Reduction (SCR) catalyst 15, and aparticulate filter 16.

The ALNC catalyst 13 preferably comprises a precious metal or acombination of precious metals, such as Platinum or Palladium, an acidicsupport material, such as the one containing alumina and silica, and azeolite material. A reductant injection system 11 is coupled to theexhaust gas manifold upstream of the ALNC. The reductant injectionsystem delivers reductant, such as fuel (HC), from the fuel tank or froma storage vessel (not shown) to the ALNC to improve its NOx conversionefficiency. System 11 may be any system known to those skilled in theart capable of delivering reductant to the NOx-reducing catalyst.Alternatively, any other means known to those skilled in the art todeliver reductant to an exhaust gas aftertreatment device may be used.

The ALNC stores hydrocarbons in the engine feedgas when exhaust gastemperatures are low, such as at engine cold start and during extendedperiods of light load operation. This prevents hydrocarbon slip into theSCR catalyst at low exhaust gas temperatures. Further, the ALNC iscapable of quick warm-up because its small size allows it to be placedclose to the engine. Once the ALNC has reached light-off temperatures,extra hydrocarbons may be injected to create an exotherm that in turnwill warm up the oxidation catalyst 14 which is located furtherdownstream. Additionally, the feedgas NOx is reduced in the ALNC in thepresence of injected hydrocarbons.

Oxidation catalyst 14 is a precious metal catalyst, preferably onecontaining platinum, for rapid conversion of unreacted hydrocarbons(HC), carbon monoxide (CO) and nitrous oxide (NO) in the exhaust gasmixture exiting the ALNC. Additionally, once the oxidation catalystreaches light-off temperatures, extra hydrocarbons can be injected intothe oxidation catalyst thus to exothermically combust and thus generateheat for fast warm-up of the urea-based Selective Catalytic Reduction(SCR) catalyst 15. Extra hydrocarbons may be supplied to the oxidationcatalyst via an injection system 11 upstream of the ALNC, or injecteddirectly into the exhaust gas stream entering the oxidation catalyst viaan additional reductant injection system (not shown). Alternatively,engine-related measures, such as, for example, in-cylinder injectionduring either or both of a power or exhaust stroke of the engine (in adirect injection engine) or any of a number of other alternatives, suchas retarding injection timing, increasing EGR and intake throttling, orany other means known to those skilled in the art to increase the HCconcentration in the exhaust gas may be used. In other words, once theALNC catalyst reaches light-off temperatures, extra hydrocarboninjection into the ALNC can be used to achieve quicker light-off for theoxidation catalyst.

The SCR catalyst 15 is, preferably, a base metal/zeolite formulationwith optimum NOx conversion performance in the range of 200-350° C.Reductant, such as aqueous urea, is stored in a storage vessel (notshown) and delivered to the SCR catalyst via a reductant injectionsystem 17. Typically, the amount of reductant injected into the SCRcatalyst is calibrated to achieve a certain reductant to incoming NOxratio.

NOx sensors, NOx, (18) upstream, and NOx₂ (19) downstream of the SCR,are coupled in the path of the exhaust gas entering and exiting the SCRcatalyst. The outputs of these sensors are read by controller 12 and maybe used to determine the NOx conversion efficiency of the SCR. If adetermination is made that the SCR performance is degraded, it ispossible to use the ALNC as a back-up NOx aftertreatment device. Undersuch circumstances, urea injection into the SCR catalyst may be reducedor discontinued, and extra reductant may be injected into the ALNC basedon an amount of NOx in the engine feedgas. The amount of NOx in theengine feedgas may be measured by an additional NOx sensor (not shown)placed upstream of the ALNC, or, alternatively, may be estimated basedon engine speed, load, exhaust gas temperature or any other parameterknown to those skilled in the art to affect engine NOx production.

Particulate filter (PF) 15 is coupled downstream of the SCR catalyst andis used to trap particulate matter (soot) generated during the drivecycle of the vehicle. The PF can be manufactured from a variety ofmaterials including cordierite, silicon carbide, and other hightemperature oxide ceramics. Once soot accumulation has reached apredetermined level, regeneration of the filter becomes necessary.Filter regeneration is accomplished by heating the filter to atemperature that will burn soot particles at a faster rate than thedeposition of new soot particles.

Therefore, according to the present invention, improved emission controlcan be achieved by placing an ALNC in addition to an oxidation catalystupstream of a urea-based SCR catalyst. The ALNC generates an exothermthat provides higher exhaust gas temperature during vehicle cold-startand light-load operation and reduces light-off time of the oxidationcatalyst resulting in faster warm-up of the SCR catalyst. Additionally,the ALNC stores hydrocarbons when the exhaust gas temperatures are low,thus preventing hydrocarbon slip from the oxidation catalyst into theSCR catalyst during the time period before the oxidation catalystreaches light-off. Also, the ALNC catalyst may serve as an auxiliaryNOx-reducing device in case the SCR catalyst performance is degraded.

In an alternative embodiment (not shown), oxidation catalyst 14 may beeliminated, and the exotherm for warming up the SCR catalyst may besupplied solely by the ALNC.

This concludes the description of the invention. The reading of it bythose skilled in the art would bring to mind many alterations andmodifications without departing from the spirit and the scope of theinvention. Accordingly, it is intended that the scope of the inventionbe defined by the following claims:

1-5. (canceled)
 6. A method for controlling a temperature of anoxidation catalyst coupled downstream of an Active Lean NOx catalyst(ALNC), comprising: providing an indication that the ALNC temperature isabove a first predetermined temperature; and in response to saidindication controlling the temperature of the oxidation catalyst byadjusting an amount of reductant in an exhaust gas mixture entering theALNC.
 7. A method for controlling a temperature of an oxidation catalystcoupled downstream of an Active Lean NOx catalyst (ALNC) during coldstart, comprising: providing an indication that the ALNC temperature isabove a first predetermined temperature; and in response to saidindication adjusting an amount of reductant in an exhaust gas mixtureentering the ALNC thereby raising the temperature of the oxidationcatalyst above a second predetermined temperature.
 8. A method forcontrolling a temperature of an oxidation catalyst coupled downstream ofan Active Lean NOx catalyst (ALNC) during cold start, comprising:injecting a predetermined amount of reductant into an exhaust gas streamentering the ALNC when the oxidation catalyst temperature is above apredetermined temperature; and increasing reductant injection into theALNC thereby causing the oxidation catalyst temperature to reach saidpredetermined temperature otherwise. 9-10. (canceled)